Significant progress has been achieved in research and development of applications that take advantage of inter-vehicle communication. Since the ITS (Intelligent Transport Systems) applications enabling various types of cooperation by inter-vehicle communication can be realized, the role of inter-vehicle communication is expected to become increasingly important in the future. However, the problem is that frequency resources allocated to inter-vehicle communication are insufficient and there is no communication band sufficient for implementing the ITS applications.
The use of a cognitive radio has been investigated as means for resolving the aforementioned problem. The cognitive radio is a technique for increasing the usage efficiency of frequency at which a wireless communication device recognizes and acknowledges the surrounding radio environment and adaptively changes the frequency or wireless system used for wireless communication. In the inter-vehicle communication, a method for using a spectrum sensing cognitive radio that performs communication at an unused empty frequency has been considered. In particular, a form in which the frequency which is allocated to, but is actually not used by a licensed user (primary user) is used by an unlicensed user (secondary user) has been considered. Such a frequency is called a secondarily usable frequency or a white space.
A method of performing spectrum sensing is used to detect the white space. Under the moving conditions such as realized in a vehicle or the like, the movement of the vehicle is followed by significant channel fluctuations. Therefore, a technique for performing adaptive sensing is needed. When one vehicle is sensed a plurality of times or a plurality of vehicles is sensed at the same time, where there is a correlation between the measurements, a diversity effect cannot be obtained. Patent Literature 1 suggests increasing the sensing accuracy by using temporal diversity, spatial diversity, or a combination thereof.
Channel fluctuations (fading) are caused by two reasons, namely, multipath fading and shadow fading. The multipath fading is a phenomenon occurring for a comparatively short time and on a comparatively short distance scale, whereas the shadow fading is a phenomenon occurring for a comparatively long time and on a comparatively large distance scale. Most research considers only shadow fading (Non-Patent Literature 1 to 3) or only multipath fading (Non-Patent Literature 4 and 5) in the cooperative spectrum sensing. In Non-Patent Literature 6, both the multipath fading and the shadow fading are considered, but the sensor is presumed to be in a fixed node, and the system cannot be directly used in the mobile environment.